Fiber optical SPR sensors have attracted much interest in recent years
for their compactness and in situ sensing capability
 [1]}, [2]}, [3]}, [4]}, [5]}, [6]}. This
fiber sensor relies on a plasmonic wave, stimulated by the evanescent
field leaked from the fiber core, to sense biochemical
interactions. There are several ways to leak an evanescent field for
chemical sensing. One popular design involves integrating the sensing
surface on the inner walls of holes in the
fiber [7]}, [8]}. These
gold film coating holes are located near the fiber core, which
facilitates a direct excitation of plasmonic waves. This closed-form
all-in-one design reduces the consumption of sample volume and has
high
sensitivity [9]}, [10]}.
However, the fabrication of such a sensor is challenging, especially
the metal film deposition inside the holes. Moreover, with this sensor
the experimentalist lacks an auxiliary method to check the metal film
and the self-assembled bio-molecular film. Another approach to leak an
evanescent field is using post-processing, for example, by drawing or
side-polishing the fiber. A fiber with a diameter on the
order of the wavelength of visible light is ready to leak an
evanescent
field [11]}, [12]};
however, this drawing process will produce a highly fragile fiber
sensor which limit their application. A robust fiber sensor can be
fabricated by side-polishing an finished
fiber [13]}, [14]}, [15]}. This
method has the advantage of simplicity, as well as allowing good
control over metal film deposition. The planarity of the sensor region
allows the direct adoption of techniques from existing prism SPR
technology. In Ref. [15]} the authors propose a
side polished fiber optic SPR sensor based on all-solid photonic crystal fiber. Since the
mechanical characteristics of the core and the cladding layer are
identical, the fabrication is simplified. The major problem with this
side-polishing sensor is that the sensitivity is limited in comparison
with the closed-form design. This problem should be addressed with
theoretical guidance. After studying the SPR theory, we suggest that
decreasing and tilting the fiber core mode index is one way to improve
the sensor performance.
